Many businesses require a number of telephone terminals to share a single directory number. The group of terminals sharing a directory number is referred to as a key-system group. A call to the shared directory number may be answered at any of the terminals in the key-system group. When a call is answered, a system connection must be established through the switching system from the originating point for the call, either a line or trunk, to the answering terminal. For a call to a non-shared directory number, this system connection is established before a call is offered to the terminal since only one terminal is addressed by the call. However, if a directory number is shared, immediately establishing connections for all terminals sharing the directory number wastes system resources since at most one terminal will answer. In addition, some system architectures may not allow for establishing multiple paths.
Establishing this connection before answer reduces the delay that a customer would otherwise experience when answering a call. Customers expect that as soon as they lift the telephone handset they may begin speaking. The answer delay which occurs if the system connection is not already established often clips the speech of the person answering the call. In a business environment, where key-system groups are often used, an executive may answer the phone simply by stating his or her name. If answer delay occurs, clipping may cause the executive's name to be unrecognizable. For example, the executive may answer the phone with "Phillips," but because of clipping from answer delay the calling party may hear only "ips." This is an extremely annoying problem. In addition, the caller will be confused by what he has heard and may be uncertain whether he has reached the correct party. Clipping is even more likely to occur when a speakerphone is used. With a traditional telephone terminal, the answering party does not begin speaking until he lifts the telephone handset from the cradle to his ear. There is no such natural delay with a speakerphone. The user expects that he can start talking over the speakerphone as soon as he presses a button to answer a call.
Establishing the system connection before offering the call to a terminal also avoids error conditions in which a call alerts at a customer's terminal but the customer's attempt to answer the call fails. In some cases the system cannot establish the connection. This happens when an error occurs or when resources are not available. If the system connection is attempted before the call is offered to the terminal, the called terminal is not rung when the path cannot be established. This is more desirable than ringing the terminal and discovering when someone attempts to answer that the system connection cannot be established.
Earlier systems attempted to solve these problems by immediately establishing a connection from the originating point for each incoming call to a predefined lead terminal in the key-system group and then offering the call to all terminals in the key-system group. In these systems, if the call is answered by the lead terminal, good performance results. However, if any terminal other than the lead terminal answers the call, answer delay may be excessive since no connections have been established for the other terminals. In addition, error conditions could occur in which a connection could not be established to one of the other terminals after an answer attempt.
The lead terminal designation is defined by static data stored in the switching system and initially set at subscription time. Changing the terminal designated as the lead terminal requires a service order. A service order is a means for the customer to request changes to static data in the switching system. Once established, the lead terminal designation is rarely changed. A number of restrictions limit the terminals in the key-system group that may be identified as the lead terminal. For example, the lead terminal must have the maximum number of call appearances for the directory number shared by the key-system group. The lead terminal designation is normally changed only when terminals are added to the group or the groups is reconfigured.
This answer delay problem is of particular concern when the key system group is distributed to a plurality of switching modules. In typical applications, it is important that the members of a key-system group can be spread across a number of switching modules to enhance reliability and provide for administrative convenience in changing group members. In such a distributed system, the answer delay problem becomes more cumbersome. In an example distributed system, a connection must be established from an originating switching module for the call through a time-multiplexed switch to the switching module to which the called terminal is connected. Further connections must be established within that switching module. These include a connection through a time-slot interchange unit to an integrated services line unit. Additional connections are required within the line unit to complete the connection to the line card connected to the called terminal. Establishing such connections after an answer request is received significantly adds to answer delay. If the earlier method of immediately establishing a system connection to the lead terminal is used, a new connection from the time-multiplexed switch to the line card must be established when an answer request is received from a terminal connected to a different switching module than the lead terminal.
Thus, a recognized problem in the art is the customer perceivable answer delays and errors that occur when an incoming call is answered at a terminal in a multi-terminal group of a switching system without a system connection initially established to the terminal.